CMS is one of the two huge detectors built to study the high-energy collisions of protons produced by the Large Hadron Collider at CERN. As all previous collider detectors, CMS is a redundant multi-purpose collection of dozens sub-detector components, which use different physics mechanisms to detect everything that comes out of the collision point, from protons to muons to photons, neutrinos (using the energy imbalance in the calorimeters), neutral hadrons.

A transverse cut-away view of the CMS detector is shown below, with the different signals that arise from the interaction of different particles. From this view you may well observe why CMS was named "compact muon solenoid": it is the huge external muon detector system (the detectors within the "red" iron slabs on the right) what gives CMS its appearance. Muons have been quite important in the detection of a Higgs boson decay signal, and would have been crucial to discover the Higgs if the elusive particle had a larger mass -in which case the decay to photon pairs would have been invisible, and the decay to two Z bosons would have been the only one granting a full mass reconstruction of Higgs candidates.

Muons are important, but they are not copiously produced in proton-proton collisions. What comes out of the interaction point is in fact, in most cases, a dijet event. Pairs of jets of light hadrons are the result of two quarks hitting each other hard, and kicking one another out of their parent proton. Quarks cannot live as free objects, and as they exit the proton the strong force pulls them back with an increasing force, causing them to decelerate and popping out of the vacuum additional quark-antiquark pairs. What we observe is a collimated stream of protons, pions, kaons: particles formed by quark-antiquark pairs or by quark triplets. These we call "jets".

The picture above is a good example of how we like to represent jets as our detector sees them: we imagine the detector is a cylinder, surrounding the interaction point and coaxial with the beam line where protons run. We then cut a side of the cylinder parallel to the beam, and unroll it. We obtain a rectangle, and proceed to plot on it bars which have a height proportional to the amount of energy that the detector measured in each location. This way the streams of particles stand up as towers, well showing that the hard interaction had the main result of producing these two phenomena on opposite sides of the interaction.

The physics of jets is anything but boring; and yet we usually discard most of these events already at trigger level, i.e. during the online data acquisition phase. The reason is that we have had a chance of studying in great detail the mechanism of jet production in past experiments. The precious bandwidth leading to the storing of all information to disk is saved for rarer events featuring particles seen less often in hadron collisions -muons, for instance. CMS can write the full reading of its detector components about 300 times a second, so a draconian selection needs to be enforced, rejecting about 20 million "uninteresting" collisions every second. A sad necessity.

But we do not discard jet events regardless: these in fact remain extremely interesting when their energy is very high ! Past experiments have not had a chance of studying events where the energy available to create new states of matter was higher than about one TeV (a thousand proton masses). With 8 TeV of center-of-mass energy the LHC can extend the reach by about a factor of four with respect to the Tevatron collider. If we have not found any new state of matter produced in quark-antiquark interactions or gluon-gluon collisions with a mass M<1 TeV we still want to see whether such things exist with masses 1<M<4 TeV!

So CMS saved to disk events with high-energy jets, and later filtered them and analyzed them, reconstructing the energy of the two main jets of particles as reconstructed by the various detector elements. Here I am not just talking about the calorimeters: CMS uses a very effective "particle flow" algorithm which reconstructs the energy of every particle making up the jet, using to the fullest the information available from the inner tracker, the electromagnetic and hadronic calorimeters, and the muon chambers.

Once the energy of the two hadronic jets is known, one computes the "dijet invariant mass": this is the mass of a hypothetical particle produced in the collision, if it decayed yielding the two observed jets of hadrons. Of course if there is no particle, but just pairs of quarks kicked out of the collision point by the strong interaction, one does not expect to observe anything else than a smoothly falling histogram of dijet invariant masses. If, however, some events do originate from the production of a new particle, they will produce an extra "bump" in the spectrum.

Above, the CMS data (black points) is compared with a Monte Carlo prediction (labeled "QCD Pythia"). Overlaid are two different possible new physics signals: the expected signal from a W' boson (in red, left) and the expected signal of a E6 diquark (in green, right). Both these signals would be detectable in the 4 inverse femtobarns of 8-TeV collisions analyzed by CMS, and are excluded by the search. The lower panel shows the residuals resulting from subtracting from the data the fitted background: there is no significant bump anywhere.

It is the search for such a bump in the spectrum that allows CMS to make a statistical inference on the existence of new particles decaying to hadronic jets. A fit to the observed spectrum can be attempted with a "theory-inspired" functional form which is featureless enough and yet captures the varying slope. By a Bayesian statistical procedure the likelihood of the data, together with a flat prior belief on the intensity of a unknown new particle signal, produces a "posterior distribution" of the possible signal strength. This is always compatible with zero, so there is no evidence of a new particle in the data. For all mass hypotheses of the new particle a 95% confidence level on the signal strength is finally derived by finding the strength below which lies 95% of the area of the posterior distribution.

The result is then a set of 95% upper limits on the cross section of the new particle production. This can be represented as a curve in the cross section versus mass graph, as in the figure on the right: all cross sections above the red curve are excluded by the observed data. By comparing the curve with the predicted cross section of new particle production -which is a quickly falling function of the invariant mass of the particle, and is here shown with two different set of dashes for two models- one can then make a statement on the excluded masses: masses smaller than the mass value where the theoretical cross section crosses the 95% upper limit are excluded by the search.

Since different hypothetical particles have different predicted production rates, there are several lower limits on the particle masses to derive from the CMS search. We learn that Randall-Sundrum gravitons must have a mass larger than 1.45 TeV if they exist; that a string resonance must have a mass above 4.78 TeV to have escaped detection; and that no Colorons are there with masses below 3.27 TeV; the limit on an excited quark are instead M>3.19 TeV. More detail is of course available in the CMS preprint.

So, all in all we learn that despite having increased the search range by a factor of four since the Tevatron era, the LHC does not see new resonances. There appear to be no fishes waiting to be pulled up in that energy range. For many, it is a disappointment; I am also disappointed in a way, but I at least did not delude myself into thinking we would have caught fishes wherever we cast the net at the LHC!

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Comments

I'm a little confused by this:

"Quarks cannot live as free objects, and as they exit the proton the strong force pulls them back with an increasing force, causing them to decelerate and popping out of the vacuum additional quark-antiquark pairs. What we observe is a collimated stream of protons, pions, kaons: particles formed by quark-antiquark pairs or by quark triplets."

If quarks cannot live as "free objects," then how can a "stream of . . . quark-antiquark pairs or . . . quark triplets" emerge from the "vacuum"? It's as if they can only live inside hadrons, but they're everywhere to be found, and "free", outside of hadrons in the so-called "vacuum."

Dear onlooker,
What I refer to when I speak of free objects is "asymptotic states". If you pull a quark out of the proton its potential energy increases linearly with the distance, such that you would need to spend an infinite amount of energy to separate it. In reality as you give more and more energy to the system you create a way out: the materialization of a brand new quark-antiquark pair from the vacuum costs an energy equal to twice the quark mass, but if the new quarks reduce the length of the extended "color string" - cutting in half the force field lines and reducing the potential energy- this occurs by force, as in any other physical system where potential energy, if allowed to, turns in other forms (kinetic, e.g., when a ball rolls down a slope).
Perhaps what confuses you is the vacuum: the vacuum is in fact not empty at all. It is a continuous bubbling of particle-antiparticle pairs, which do not come into real existence because they do not have the energy to convert in real mass. Heisenberg's uncertainty principle explains that such virtual particles can indeed exist for a time δt small enough that the "loan" of energy they require to the vacuum satisfies δEδt

.... Whoops the "less than" sign was interpreted as an html tag and the rest of my message got cut.
Anyway dE dt less than h/2 pi, and h is Planck's constant. So for unimaginably small time intervals these pairs can borrow the energy dE to pop into brief existence.
What I think confuses you is the notion that quarks could exist in this vacuum. But note: quarks carry the charge if the strong interaction, colour, which the vacuum does not. Since colour is locally conserved, no less and no more than electric charge, there is no way for a single quark to pop into existence. Only particle-antiparticle pairs can, because collectively they carry the same quantum numbers of the vacuum.
Hope this helps,
T.

You say there is no "color" in the vacuum. This is because the vacuum carries EM charge, I suppose. Yet, this makes it even harder to understand why 'quarks', whether 'quark' or 'anti-quark,' would somehow emerge from the vacuum.

I'm familiar with the notion of "virtual" particles and the vacuum and Heisenberg's Uncertainty Principle. And these "quark-anti-quark" particles are certainly "virtual" particles. I guess my question is this: you say that you can't dislodge a quark from a proton/neutron; yet, you say that quark-anti-quark particles are streaming away. But how in the world would you know what a "quark" looks like if one has never been freed from a hadron (I'm familiar enough, qualitatively, with asymptotic freedom and such).

I think were dealing here with "energetic" particles that have the right amount of "mass" (actually energy in ev units). So, maybe I'm asking how it is you know that you're looking at these things? What is it about them that lets you know what they are? Do they have a particular signature (other than their energy)? How do you know, IOW, that you're not dealing with massive bosons like in the weak force?

Sorry to be asking you to pin this down, but what is clear to me is that I'm missing something here. Thanks again for your previous answer. Ciao.

the only particle with the quantum numbers of the vacuum is the Higgs boson. No charge, no color charge. The vacuum cannot be charged! Charge is an additive quantum number, so that zero is equal to +1 -1: you can "pop" out of the vacuum a positive charge only if you simultaneously pop out also a negative charge, in the same space-time position.

We know pretty well many properties of quarks, but we do not know how they "look like", if you want, precisely because of infrared slavery - their confinement conditions.

You ask how it is that we know we are looking at those things. So imagine you throw a black garbage bag against another black garbage bag, with very high energy. You see a mess coming out, but in particular you observe pieces of broken glass and smell whiskey. You may infer that one of the bags contained an empty bottle of bourbon, right ?

Leaving empty analogies away, we have a model, the model is predictive, and so until we find a contradiction we believe it. The model says hadrons are made of quarks, explains how these are bound together, what is the potential energy of quark-gluon configurations, and the phenomenology one should expect for bound states of different kinds. That is just good science, no less and no more than having a model for how electrons move in a piece of metal. We know we can transport electricity through the piece, and we know a whole lot of things about the different properties of conductors; however we are only basing our knowledge on a model. The evidences for it are overwhelming, but it is just a model. That is how we do business.

Believe it or not, I'm better understanding all of this. And I know I've already pestered you enough. Ma . . . .

You use the analogy of "the smell of whiskey and broken glass." From "Gauge Theories in Particle Physics", I know that there is experimental evidence of resonances of sorts in high-energy hadron collisions which are associated with quarks.
So, by 'the smell of whiskey', you might mean the energy level of a quark. But what might the 'pieces of broken glass' correspond to? I would just suppose that it has to do with the kinds of decay particles seen in the di-jet. Is that correct?

We have models that allow us to calculate how a quark will hadronize into a stream of particles. We can compare the observed distribution of produced bodies with what we simulate, and if we get a close match we have a working hypothesis on the details of the physics.Observables we can measure and compare are: the kinds and number of particles produced, their energy, and the transverse momentum distribution relative to the jet axis; the jet mass (total invariant mass of produced bodies), the fraction of pi-zeros (which experimentally give electromagnetic component in the measured jet because pizeroes decay to photon pairs and photons interact in the first layers of the calorimeter, yielding what we call "electromagnetic component" of the energy stream.

"Quarks cannot live as free objects, and as they exit the proton the strong force pulls them back with an increasing force, causing them to decelerate and popping out of the vacuum additional quark-antiquark pairs. What we observe is a collimated stream of protons, pions, kaons: particles formed by quark-antiquark pairs or by quark triplets."

If quarks cannot live as "free objects," then how can a "stream of . . . quark-antiquark pairs or . . . quark triplets" emerge from the "vacuum"? It's as if they can only live inside hadrons, but they're everywhere to be found, and "free", outside of hadrons in the so-called "vacuum."

The only meaningful vacuum is the QCD vacuum inside of the proton, and the q-qbar pairs which appear there are intimately tied to the gluonic field configuration that binds the valence quarks. No meaning can be attached to the q-qbar content of the vacuum outside of the proton, unless you want to discuss what happens at the event horizon of a black hole.

Well, if you believe the vacuum outside of the proton has a scalar q-qbar condenstate which fills all of space-time you and breaks the chiral symmetry of the quarks, then according to Nambu, pions and hadrons are just traveling excitations and solitons of this soup. But I'm not sure how you prove this picture is experimentally distinguishable from pions and hadrons just being bound-states of quarks, with the vacuum outside empty of structure.

Tommaso Dorigo: CMS uses a very effective "particle flow" algorithm which reconstructs the energy of every particle making up the jet, using to the fullest the information available from the inner tracker, the electromagnetic and hadronic calorimeters, and the muon chambers.

Do you have visual examples of this particle flow?

As a side note it is interesting to see how theoretical propositions are being adapted to the experiments, Randall-Sundrum gravitons, string resonance and may I say on a most general level how, "we imagine the detector is a cylinder, surrounding the interaction point and coaxial with the beam line where protons run. We then cut a side of the cylinder parallel to the beam, and unroll it. We obtain a rectangle, and proceed to plot on it bars which have a height proportional to the amount of energy that the detector measured in each location. This way the streams of particles stand up as towers, have https://encrypted-tbn3.gstatic.com/images?q=tbn:ANd9GcQgPQ4Aa16DByAKB-Ir... -comparative views in the theoretical world.- https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcRVgZys-ng9ShSZOJkn...

I have "Gauge Theories in Particle Physics", and in Chapter 1 they show a kind of energetic "tower," when describing quarks and such. I imagine this is the kind of "tower" you're speaking of. So there are energetic perturbations and decays, and these line up with values of masses for certain particles. So, actually, we see 'energy', and not the actual, but a 'virtual', particle.

I've reread everything you've written in response to my queries, and I think I understand everything right now.

The basic distinction is that between an actual particle (having mass) and a "virtual" particle which is effected in the vacuum as a result of disturbing the proton. So---in my way of understanding this---we're seeing the creation of virtual particles as a result of a quark being 'bounced outwardly' from the basic proton structure. Gratie per tutto.

Tommaso Dorigo:The physics of jets is anything but boring; and yet we usually discard most of these events already at trigger level, i.e. during the online data acquisition phase. The reason is that we have had a chance of studying in great detail the mechanism of jet production in past experiments.

So you have in essence arrived at a QGP position on what is capable in collision process?

Tommaso Dorigo:Searching for new particles decaying to jet pairs is one quite important part of the investigations of the high-energy frontier at hadron colliders. A number of new physics objects would, if they existed, be first detected in that final state: this is due to the fact that in hadron collisions one usually produces with large rates objects that feel the strong interaction.

Can you comment on the 23 February 2013 CDF Wine and Cheese talk by Trovato entitled:
"Update on dijet mass spectrum in W + 2 jets events" ?

Francis the Mule posted today (28 February 2013) about it,
saying in part (based in part on Google translation to English):
"... The Monte Carlo technique used to adjust the triggers used in the identification of the jets,
confuses some background noise types with fake leptons especially electrons ...
readjusting event selection techniques [causes] the W+jj anomaly to disappear [so that] the data correspond to the predictions of the standard model ...".

Could some similar problems be at the root of differences in Higgs mass measured by ATLAS and CMS in diphoton and ZZ4l channels ?

If so, bearing in mind it took 2 years for CDF to find the root of the W+jj discrepant results, is it likely that a similar resolution to the Higgs mass discrepancies might be found and understood prior to the end of the shutdown of the LHC around 2015 ?

the W+jj "signal" of CDF was an artifact of a badly estimated background. There can be no parallel with the Higgs search in ATLAS, where the background is not estimated (in the H->gg case) but just fit with a number of possible shapes; or in the H->ZZ case, where it is basically uninfluent in the fit mass (which depends only on the energy scale of electrons and muons).

The Atlas "twin peaks" effect is only the result of fluctuations or energy scale issues - if it is not new physics ;-)